Electronic circuit including operational amplifier and clamping circuit

ABSTRACT

An electronic clamping circuit is provided in one preferred embodiment, the clamping circuit includes a pair of series-connected diodes, both having the same bias, which are shunted across a feedback path of a transimpedance amplifier circuit. A capacitive element is connected to a node in-between the diodes and a potential (e.g., ground). The arrangement of the diodes and capacitive element serve to keep the amplifier circuit&#39;s operation within its linear limits without severely degrading its bandwidth.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a clamping circuit and an electronicsensor which uses same. Particular utility for the present invention isfound in the area of electromagnetic radiation sensing apparatus,although other utilities are contemplated for the present invention,including other types of sensing apparatus. Additionally, the clampingcircuit of the present invention may be used in many different types ofelectronic devices, and thus, although it will be described inconnection with use in sensing apparatus, it should not be viewed asbeing limited solely to use in this one type of apparatus.

2. Brief Description of Related Prior Art

FIG. 1 is a schematic diagram of a conventional electromagneticradiation sensor 10. Sensor 10 includes an electronic transimpedanceamplifier 12 that comprises an operational amplifier 16 having invertingand non-inverting inputs 18, 20, respectively, and output 28. Theinverting input 18 of the operational amplifier is connected via a highimpedance feedback path 22 to the output 24 of the amplifier 16, and isalso conected, in parallel, to an electromagnetic sensitive photodiode26 whose anode is connected to ground and cathode is connected to node54. Typically, the photodiode 26 is made from silicon. Feedback path 22consists of a resistor 30 (typically having a very large resistancevalue, e.g., on the order of 10⁹ ohms) connected between the output 28and the inverting input 18 of the amplifier 16 via node 54. Thenon-inverting input 20 of the amplifier 16 is connected to groundpotential.

In normal operation (i.e., when amplifier 16 is unsaturated), photodiode26 responds to incident electromagnetic radiation within its frequencyband of sensitivity (e.g., the optical frequency band) by generating aphotocurrent I_(p) that is related to the strength of said radiation. Inresponse to the photocurrent I_(p) generated by the photodiode 26, theoperational amplifier 16 supplies feedback current I_(f) through thefeedback path 22. In this configuration, operational amplifier 16 isconfigured to keep the voltage levels of the inverting and non-invertinginputs 18, 20, respectively, equal to each other. Thus, because thenon-inverting input of the amplifier 16 is held at ground potential, theoperational amplifier 16 will seek to supply appropriate feedbackcurrent I_(f) to negate the photocurrent I_(p) generated by thephotodiode 26 so as to prevent a potential difference from beinggenerated across photodiode 26, and to thereby force the inverting input18 of the operational amplifier to remain at ground potential. Thus, thefeedback current I_(f) generated by the amplifier 16 is equal inmagnitude to the photocurrent I_(p) generated by the photodiode 26, butopposite in direction thereto.

Thus, in normal operation of sensor 10, the magnitude of the outputvoltage V_(out) of the operational amplifier 16 is equal to themagnitude of the photocurrent I_(p) multiplied by the resistance of theresistor 30. Thus, during said normal operation, the magnitude of theoutput voltage increases in proportion to the magnitude of thephotocurrent I_(p), and therefore, is indicative of the magnitude ofelectromagnetic radiation incident to diode 26.

Unfortunately, operational amplifier 16 has a limited output voltagerange, and typically will saturate when the magnitude of the outputvoltage of the operational amplifier is at or near the magnitude of thepower supply voltage being supplied to the operational amplifier (e.g.,15 V). When the operational amplifier is saturated, the magnitude ofoutput current generated by the operational amplifier remains constantregardless of further increases in photocurrent generated by thephotodiode. Thus, once sufficient photocurrent is generated to cause theoperational amplifier to saturate, the operational amplifier becomesunable to generate additional feedback current to compensate for anyfurther increases in photocurrent. This can cause the voltage level ofthe inverting input 18 of the operational amplifier to rise to a levelequal to the uncompensated photocurrent multiplied by the impedance ofthe photodiode 26. Disadvantageously, this can de-stabilize theoperation of the operational amplifier.

Also, since the photodiode 26 has an inherent parasitic capacitance, thepotential difference generated across the photodiode 26 as a result ofthe aforesaid phenomena can cause the photodiode to become charged. Thecharge stored in the photodiode 26 is then discharged when the strengthof the incident electromagnetic radiation falls below the levelsufficient to cause saturation of the operational amplifier.Disadvantageously, discharge of the charge stored in the photodiode 26artificially increases the photocurrent supplied by the photodiode abovethe amount that is truly indicative of the strength of incidentelectromagnetic radiation. This can cause the output voltage generatedby the operational amplifier to not accurately indicate the strength ofincident radiation striking the photodiode. Indeed, it has been foundthat after significant periods of charging, if the incidentelectromagnetic signal exciting the photodiode 26 is suddenly removed,the aforesaid discharging phenomena may keep the operational amplifier'soutput voltage at saturation level for as long as several seconds.

One conventional attempt to solve this problem is embodied in sensor 50of FIG. 2. Sensor 50 includes all of the elements of sensor 10 of FIG.1, but also includes a clamping diode 52 shunted across the feedbackresistor 30 of the feedback path 22 and forward biased in the samedirection as the flow of feeback current I_(f). In operation of sensor50, when the output voltage of the operational amplifier 16 increases,the impedance of the clamping diode 52 decreases and thereby reduces thetotal resistance of the parallel combination of the feedback resistor 30and diode 52. This permits an increased amount of total current to flowfrom output 28 to node 54 to cancel out photocurrent I_(p). Using thistechnique, it is possible to ensure sufficient current flow to node 54from output 28 to negate any expected magnitude of photocurrent.

Unfortunately, however, the technique illustrated in FIG. 2 has at leastone significant drawback. Diode 52 has its own inherent parasiticcapacitance (typically at least 10 pF) which parasitic capacitancedeleteriously effects the bandwidth and time constant of the sensor 50of FIG. 2 when compared to the sensor 10 of FIG. 1. Indeed, it has beenfound that the inclusion of clamping diode 52 in sensor 50 can decreasethe bandwidth and increase the time constant of sensor 50 by more thanan order of magnitude when compared to the sensor 10 of FIG. 1.

An example of a conventional clamping circuit is described in U.S. Pat.No. 4,578,576 to Wheeler et at. Unfortunately, this patent suffers fromthe aforesaid and/or other disadvantages and drawbacks of the prior artdiscussed above.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a clamping circuit andsensor using same that overcome the aforesaid and other disadvantagesand drawbacks of the prior art. One preferred embodiment of the clampingcircuit of the present invention essentially comprises a pair of diodesconnected together in series and being biased in the same direction.Another diode is shunted across one diode of the pair of diodes and isreversed biased compared to the one diode. A capacitive element connectsan anode of the other diode to a ground potential. Preferably, thecapacitive element has a capacitance C, that substantially satisfies therelationships C≧10*Cd wherein Cd is the parasitic capacitance of thediodes. Also preferably, all of the diodes are silicon carbide diodes.

The clamping circuit of this embodiment may form part of atransimpedance amplifier-based sensor. In such an arrangement, thetransimpedance amplifier includes a feedback path connecting an input ofthe amplifier to an output of the amplifier. A sensor circuit isprovided for supplying current to the input of the amplifier in responseto a signal applied to the sensor circuit. Preferably, the sensorcircuit comprises a silicon carbide photodiode or other high impedancephotodiode connected to a transistor-based input stage. The pair ofseries-connected diodes of the clamping circuit are shunted across thefeedback path of the amplifier.

Advantageously, it has been found that by incorporating the clampingcircuit of the present invention in a transimpedance amplifier-basedsensor, it is possible to ensure that sufficient current flow throughthe amplifier's feedback path exists to negate any expected value ofphotocurrent generated by the sensor circuit, without substantiallydeleteriously effecting the bandwidth and time constant of the sensor.

Also the silicon carbide diodes used in the sensor and clamping circuitof the present invention exhibit much better temperature vs. leakagecurrent characteristics than conventional silicon diodes of similarconstruction (e.g., in some cases, more than several orders of magnitudebetter temperature vs. leakage current characteristics than silicondiodes). Advantageously, this permits the sensor and clamping circuit ofthe present invention to exhibit much better performance in hightemperature environments than is possible according to the prior art.

These and other features and advantages of the present invention willbecome apparent as the following Detailed Description proceeds and uponreference to the Drawings, wherein like numerals depict like parts, andin which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of a conventional transimpedanceamplifier-based electromagnetic radiation sensor;

FIG. 2 is a schematic circuit diagram of another conventionaltransimpedance electromagnetic radiation sensor;

FIG. 3 is a schematic circuit diagram of one preferred embodiment of anelectromagnetic radiation sensor with a clamping circuit according topresent invention;

FIG. 4 is a schematic circuit diagram of another preferred embodiment ofan electromagnetic radiation sensor with a clamping circuit according tothe present invention;

FIG. 5 is a schematic circuit diagram of yet another preferredembodiment of an electromagnetic radiation sensor with a clampingcircuit according to the present invention;

FIG. 6 is a graph illustrating the frequency response of the prior artsensors shown in FIGS. 1 and 2; and

FIG. 7 is a graph illustrating the frequency response of the sensorshown in FIG. 4.

It will be appreciated by those skilled in the art that although thefollowing Detailed Description will proceed with reference being made topreferred embodiments and methods of use, the present invention is notintended to be limited to these preferred embodiments and methods ofuse. Rather, the present invention is of broad scope and is intended tobe limited only as set forth in the accompanying claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3 is a schematic diagram of one preferred embodiment of anelectromagnetic radiation sensor 200 with clamping circuit according tothe present invention. Sensor 200 includes sensor circuit 202, clampingcircuit 210, and conventional transimpedance amplifier 12 (whichpreferably is of the type described previously in connection with FIGS.1-2). Sensor circuit 202 preferably comprises a conventional siliconcarbide photodiode 204 for generating and supplying electricalphotocurrent I_(p) to the transimpedance amplifier 12 in response toincident electromagnetic radiation within the band to which the diode204 is sensitive (e.g., the optical frequency band). Alternatively,photodiode 204 may be replaced with a photodiode constructed fromsilicon or other high impedance material configured to generate andsupply the photocurrent to the amplifier 12 in response to incidentelectromagnetic radiation. Preferably, the anode of photodiode 204 isconnected to ground and its cathode is connected to node 54 of amplifier12.

In accordance with this embodiment 200 of the present invention,clamping circuit 210 comprises a pair of silicon carbide diodes 212, 214which are connected in series, and are shunted across the resistor 30 offeedback path 22 of the transimpedance amplifier 12. Diodes 212, 214 arebiased to conduct current in the same direction (indicated in FIG. 3) asthe feedback current I_(f) flowing through the feedback path 22. Diodes212, 214 function in a manner substantially similar to clamping diode 52of the conventional sensor 50 to ensure sufficient current flow to node54 from output 28 to negate any expected magnitude of photocurrentI_(p).

Clamping circuit 210 also includes a capacitive element (e.g., a ceramiccapacitor) 220 which is connected to node 216, in-between the diodes 212and 214, and to ground. Capacitor 220 acts as a low resistance shuntfrom node 216 to ground potential for high frequency signals passingthrough clamping circuit 210, and reduces the total effective parasiticcapacitance of the parallel combination of the pair of diodes 212 and214 and the feedback resistor 30. Preferably, diodes 212 and 214 arechosen so as to have substantially equal parasitic capacitances Cd,feedback path 22 is constructed so as to have as small a parasiticcapacitance Cf as is practical, and capacitor 220 is chosen so as tohave a capacitance C, such that C≧10*Cd. Advantageously, it has beenfound that by constructing sensor 200 in this manner, the totaleffective parasitic capacitance of the parallel combination of thediodes 212, 214 and the resistor 30 may be reduced to such an extentthat the bandwidth and time constant of sensor 200 may approach those ofthe conventional unclamped sensor 10 shown in FIG. 1, without sufferingfrom the aforesaid and other drawbacks of the conventional sensor 10 ofFIG. 1.

Also the silicon carbide diodes 204, 212, and 214 used in sensor 200exhibit much better temperature vs. leakage current characteristics thanconventional silicon diodes of similar construction (e.g., in somecases, more than several orders of magnitude better temperature vs.leakage current characteristics than silicon diodes). Advantageously,this permits the sensor 200 to exhibit much better performance in hightemperature environments than would be possible if the diodes 204, 212,and 214 were conventional silicon diodes.

FIG. 4 is a schematic circuit diagram of another preferred embodiment ofa sensor with clamping circuit according to the present invention.Unless otherwise specifically stated to the contrary, it should beunderstood that sensor 200' of FIG. 4 comprises the same elements andoperates in the same way as sensor 200 of FIG. 3. Sensor 200' includesclamping circuit 210' which comprises a third silicon carbide diode 218shunted across one diode 214 of the pair of diodes 212, 214. Diode 218is reversed biased compared to diode 214, so as to permit current flowthrough diode 218 in a direction opposite to that permitted by diodes212 and 214. Preferably, diode 218 is substantially identical inconstruction to diodes 212 and 214, and also exhibits parasiticcapacitance Cd. Diode 218 acts to clamp undesired undershoot generatedwhen the magnitude of the photocurrent changes suddenly. Advantageously,this improves the settling time of the sensor 200'.

FIG. 5 is a schematic circuit diagram of another preferred embodiment ofthe sensor with clamping circuit according to the present invention.Unless otherwise specifically stated to the contrary, it should beunderstood that sensor 300 of FIG. 5 comprises the same elements andoperates in the same way as sensor 200' of FIG. 4. In addition to theelements of sensor 200', sensor 300 includes a MOSFET input stage 302connecting sensor circuit 202 to operational amplifier 16. Input stage302 comprises a differential amplifier formed from a dual P-channelMOSFET 312, one of whose gates is connected to the cathode of thephotodiode 204 of the sensor circuit 202, and the other of whose gatesis connected to ground potential. The sources and drains of the MOSFET312 are connected via resistors 304, 306, and 310 to positive andnegative power sources V+ and V-, respectively. Preferably, theseresistors 304, 306, and 310 are chosen so as to bias transistor-basedamplifier 312 to operate in its linear region. The inputs 18, 20 of theoperational amplifier 16 are connected to the drain outputs of thetransistor amplifier 312. Preferably, in this embodiment the resistor 30has a resistance of 3×10⁹ ohms and parasitic capacitance of 1 picofarad,capacitor 220 has a capacitance C of 0.01 microfarads, and diodes 212,214, and 218 each have a parasitic capacitance Cd of 10 picofarads.

EXAMPLE

In order to demonstrate the unique advantages of the present invention,frequency responses of the conventional sensors 10, 50 illustrated inFIGS. 1 and 2 were simulated using a conventional circuit simulationcomputer program. In said simulations, the resistor 30 was given aresistance of 10⁹ ohms and a parasitic capacitance of 1 picofarad, andthe diode 52 was given a parasitic capacitance of 10 picofarads. Plotsof the simulated frequency responses of the sensors 10, 50 are shown inFIG. 6, and referred to as 400 and 402, respectively. As can be deducedfrom simulated frequency response curves 400 and 402, the bandwidths ofsensors 10 and 50 are 160 Hz and 15 Hz, respectively. Also, the timeconstants of sensors 10 and 50 are 1 millisecond and 11 milliseconds,respectively.

The frequency response of sensor 200' was simulated twice using the sameconventional circuit simulation computer program used to simulate thefrequency responses of conventional sensors 10 and 50. In saidsimulations, the resistor 30 was given a resistance of 10⁹ ohms and aparasitic capacitance of 1 picofarad, and diodes 212, 214, and 218 wereeach given a parasitic capacitance of 10 picofarads. In the first of thetwo simulations, the capacitor 220 was given a capacitance of 200picofarads, while in the second simulation the capacitor was given acapacitance of 1000 picofarads. Plots of these two simulations are shownin FIG. 7, and referred to as 500 and 502, respectively. As can bededuced from simulated frequency response curves 500 and 502, sensor200' exhibits bandwidths of 85 Hz and 140 Hz when the capacitor 220 hascapacitances of 200 picofarads and 1000 picofarads, respectively.Additionally, sensor 200' exhibits time constants of 1.9 millisecondsand 1.1 milliseconds when the capacitor 220 has capacitances of 200picofarads and 1000 picofarads, respectively.

Thus, it is evident that there has been provided in accordance with thepresent invention a clamping circuit that fully satisfies the aims andobjectives hereinbefore set forth. It will be apparent to those skilledin the art that many modifications, alternatives, and variations ofthese preferred embodiments are possible without departing from thepresent invention. Therefore, the present invention is intended to beviewed quite broadly, as embracing all such modifications, alternatives,and variations, and as being limited only as set forth in theaccompanying claims.

What is claimed is:
 1. A circuit comprising, an operational amplifierhaving a non-inverting input coupled to ground potential, an invertinginput for receiving an input signal, and an output; a resistor coupledbetween the inverting input and the output; a first diode whose cathodeis coupled to the inverting input; a second diode whose cathode iscoupled to an anode of the first diode, said second diode also having ananode coupled to the output; and a capacitor coupled between the anodeof the first diode and said ground potential.
 2. A clamping circuitaccording to claim 1, wherein at least one of the diodes comprises asilicon carbide diode.
 3. A circuit according to claim 1, wherein saidfirst and second diodes are shunted across said resistor.
 4. A circuitaccording to claim 1, further comprising a third diode shunted acrosssaid second diode and reversed biased compared to said second diode. 5.A circuit according to claim 1, wherein said capacitor is also coupledto the cathode of said second diode.
 6. A circuit according to claim 5,farther comprising a third diode having a cathode coupled to saidoutput, said third diode also having an anode coupled to said capacitor.7. A circuit according to claim 1, wherein each of said diodes is asilicon carbide diode.
 8. A circuit according to claim 1, wherein eachof said diodes has a parasitic capacitance equal to Cd, and saidcapacitor has a capacitance that is equal to or greater than ten timesCd.
 9. A circuit according to claim 8, further comprising a third diodeshunted across said second diode and reversed biased compared to saidsecond diode, said third diode also having a respective parasiticcapacitance equal to Cd.